1. Field of the Invention
The present invention relates to a solid-state image sensor in which a plurality of pixels each having a photoelectric converting element are arranged in a matrix form.
2. Related Art
In recent years, MOS (CMOS) image sensors have been actively developed. In particular, with miniaturization (a reduction in design rules) of a semiconductor process, single-plate color image sensors each having 5 million or more pixels at a pixel pitch of, e.g., 2.5 μm have been commercialized.
This type of MOS image sensor generally includes a color filter having a Bayer arrangement where one red (R) pixel, one blue B pixel, and two diagonally arranged green (G) pixels are provided in a pixel block formed of 2 rows and two columns. The reason why two pixels G are provided in the pixel block is that a photoelectric converting element has a high sensitivity with respect to green. Therefore, the green pixel is used as a pixel that acquires luminance (brightness) information.
With tendencies of an increase in pixels and miniaturization of pixels involved by a reduction in design rules of a semiconductor device, a need for realizing a wide dynamic range (WDR) has been increased. In recent years, various technologies have been proposed to avoid saturation on a high-illuminance side in particular. However, an expansion of a dynamic range toward a low-illuminance side, i.e., a reduction in a minimum object illuminance requires an improvement in an SNR on each pixel, and realization is difficult. A trend in miniaturization of pixels proceeds to a level of 1.7 μm in terms of a pixel size and 1 μm or below in terms of an aperture area. When such a pixel size is adopted, a fluctuation in incident light becomes prominent, and an amount of incident light is rapidly decreased beyond a reduction ratio of a pixel area due to image blurring (a diffraction limit). Therefore, a measure of expanding a dynamic range toward a low-illuminance side, i.e., improving an SNR is required.
Various kinds of technologies that can suppress deterioration in color reproducibility even if a pixel size is reduced have been proposed (see, e.g., JP-A 2004-304706 (KOKAI) and JP-A 9358/1996 (KOKAI)).
JP-A 2004-304706 (KOKAI) discloses a technology of arranging a white color used as a luminance signal in right, left, up and down directions with a green color being set at the center, thereby assuring a quantity of signal electric charges of the luminance signal. In case of JP-A 2004-304706 (KOKAI), a pixel block formed of four rows and four columns is judged as a unit, and there is a problem that it takes much time for the signal processing since the pixel block unit is large. Further, special signal processing is not carried out with respect to a pixel having a low luminance, and hence the pixel having the low luminance may be submerged in noise.
Furthermore, JP-A 9358/1996 discloses a technology of performing signal processing for alignment of color filters in such a manner that a sum total of spectral sensitivities of all pixels becomes R:G:B=2:3:1. However, even in case of JP-A 9358/1996, a pixel having a low illuminance is not taken into consideration. Therefore, an SNR of the pixel having the low luminance may be deteriorated.